In the past two weeks, support has fallen sharply in Iowa for Republican presidential hopeful Newt Gingrich. NBC's Chuck Todd reports.

By NBC's Mark Murray

Former Massachusetts Gov. Mitt Romney and Texas Rep. Ron Paul are running neck-and-neck in Iowa, former Pennsylvania Sen. Rick Santorum is surging and former House Speaker Newt Gingrich collapsing just four days before the state's Jan. 3 caucuses, according to a new NBC News-Marist poll.

Romney drew the support of 23 percent of likely caucus-goers in Iowa – identified based on interest, chance of voting and past participation – ahead of Paul, at 21 percent.

They are followed by Santorum at 15 percent, Texas Gov. Rick Perry at 14 percent, Gingrich at 13 percent and Minnesota Rep. Michele Bachmann at 6 percent.

The Republican presidential hopefuls are in high gear with just days left until the Iowa caucuses. NBC's Peter Alexander reports.

The poll numbers, which are similar to those published in a recent CNN/Time survey, represent a reversal of fortune for Gingrich, as well as an improvement for Santorum and (to a lesser extent) Perry. The NBC-Marist poll conducted in late November had Gingrich in the lead among likely caucus-goers at 28 percent, Romney and Paul tied at 19 percent, Perry at 10 percent, Bachmann at 7 percent and Santorum at 6 percent.

“More than half of [Gingrich’s] support has evaporated,” said Lee Miringoff, the director of the Marist College Institute for Public Opinion, which conducted the survey.

Negative advertising hits Gingrich 'on the chin'Miringoff adds that the millions of dollars in negative TV ads targeting Gingrich – from a pro-Romney Super PAC and the Paul campaign – have played a major role in this erosion, with 35 percent of likely caucus-goers now saying he’d be unacceptable as the GOP’s nominee. That’s a 19-point increase from last month.

"The fight I'm in with Romney is exactly the fight that Reagan was in with the establishment in '80," GOP presidential hopeful Newt Gingrich tells NBC's Chuck Todd in a one-on-one interview.

What’s more, only 6 percent in the survey identify Gingrich as the “true conservative” in the Republican contest.

“He took it on the chin,” Miringoff says of the negative advertising campaign, which has questioned Gingrich’s conservative credentials and tied him to Washington.

Splintered Tea Party supportAlthough just 7 percent of likely Iowa caucus-goers believe that Romney is the true conservative in the GOP field, he has two variables working in his favor, according to the poll. One, only 21 percent of likely caucus-goers say he’s unacceptable as the Republican nominee (compared with 35 percent for Gingrich and 41 percent for Paul).

And two, the conservative vote appears to be splintering between the various candidates, and is no longer coalescing around a single Romney challenger.

The wild card in this race has been and continues to be Ron Paul, the Libertarian who has a growing following inside the Republican Party. NBC's Chuck Todd has more.

Last month, Gingrich had a large lead over Romney (and the other GOP rivals) among Tea Party supporters.

But in this new poll, Tea Party supporters – who make up about half of all likely caucus-goers – are divided.

“This is the Romney dream scenario,” Miringoff says. “When you look at the Tea Party and conservatives, they are all splintered.”

Obama’s approval rating ticks up in IowaThe poll also shows an improvement in President Barack Obama’s approval rating in Iowa.

Forty-five percent of registered voters in the state approve of him, while 43 percent disapprove.

Last month, those numbers were upside down, with 43 approving and 46 disapproving.

Former Pennsylvania Senator Rick Santorum vaults past Newt Gingrich and into third place in the GOP presidential race in Iowa, according to a new poll. Santorum talks to TODAY's Savannah Guthrie about the surge, his conservative values and why he can beat Barack Obama in the general election.